Ok so there's a lot open for discussion there I think.
Sin isn't "you're a bad person" sin is "you're missing the mark." Now, in common parlance sin is associated with "you're a bad person" but that's not the fault of doctrine, that's the fault of people (if you ask me: being lazy at best or malicious at worst with the meaning of the word) - again, because people miss the mark. The interesting thing is that traditionally you don't punish people for missing the mark - you train them to get better at hitting the mark.
Things like slavery, oppression, bigotry, deviant behavior (sexual or otherwise) are all missing the mark.
Some things are clearly bad in that they obviously harm other people, like brutality, meanness, withholding food, and the like.
Other things don't appear to be harmful but they are still missing the mark. Those things aren't about harm so much as they are a corruption of the intent: there is something better. I'd say most sexual sins are in this arena - sexual expression is arguably better if in the context of a mutually respectful, typically1 procreative relationship.
I fully agree though that many modern Western "Christians" completely forget the Bible opens with the statement that we live in a fallen world - even the world itself makes life "hard" with weeds and death and striving - even striving "between man and woman." And a big part of the original sin is acting like we are gods, deciding we know what's best; basically, pride. But everything is an imperfect reflection of its intent.dis
Many modern Western "Christians" also fall into the same trap as the Pharisees - incorrectly associating righteousness with behaving in a certain way and claiming to know what's best.
My take on sexual and identity issues is like Paul says: everything is permissible, but not everything is beneficial. And this is on an individual basis, not a global statement (e.g., "some people are fine eating this food, others aren't; treat each accordingly.") And on the flip side, what's beneficial for a particular individual, isn't always beneficial for society as a whole. Even beyond that, there's the fundamental question of - does any world view promote the glory of God, or the glory of Mankind? This is a test even for many things claiming to be "Christian" - because they really don't promote the glory of God.
1 By 'typically' here I mean 'as a member of the type or class of' - that is, procreation is through male types mating with female types, even if particular members of those types are physiologically incapable of procreation.